The invention relates to a method for quantitatively estimating defects present in objects or structures, from measurements of physical quantities carried out at the surface, in order to enable a non-destructive evaluation (NDE).
At present, non-destructive control devices generally require the illumination of an object by a wave characterised by one or more physical quantities (magnetic, electric, electromagnetic field, ultrasounds, thermal infrared, X-rays or TeraHz waves, etc.). Said physical quantity, which can be qualified as primary, enters into interaction with the object observed (target), and may be directly behind the phenomenon or the image obtained (absorption of X-rays in the different parts of the human body passed through, for example), or give rise to a secondary quantity. The case of Foucault current imaging may for example be cited, in which the incident electromagnetic wave creates induced current sheets in the material (conductor): it is these current sheets that will be perturbed by a defect, characterised in this case by a variation in the electrical properties of the material (conductivity, permeability, permittivity).
The image of these perturbations is generally obtained by a series of detectors placed on the surface and which give in the best of cases (absence of diffraction, uniform ‘illumination’ of the object) an image resulting from the superposition of all the properties of the object, throughout its thickness. The simplest example is again that of X-ray radiography in which the image results from all of the parts of the body traversed by the waves, which makes the interpretation difficult. To obtain a 3-dimensional image, it is necessary to ‘enrich’ all of the signals perceived: this is the X-ray scanner, where N images are obtained for N different observation angles. This case is simpler than most non-destructive control (NDC) problems: absence of diffraction, and access to the two ‘faces’ of the object to be imaged.
In industrial NDC, there is very rarely access to the two faces of the object or the structure to be controlled, and the use of this type of technique is then very often impossible: it is necessary to resort to the use of Foucault currents, ultrasounds, infrared sensors, etc. These sensors give images very far removed from the real shape of the defects for two major reasons:                on the one hand, there is a superposition of the images observed (there may be several cracks buried under the inspected surface), and        on the other hand, the image may result from the perturbation of the secondary quantity.        
For example, FIG. 1 shows the image of the real part (Real) and the imaginary part (imag) of a magnetic field on the surface of a conductive material (2017A, formerly Au4G) comprising an emerging crack. This image has been obtained with a Foucault current imager with magneto-optic detection (ECI) as described in the documents WO2005/001467 and WO 2007/135265, to which it is possible to refer for further information. The axes X and Y are in tenths of millimeters.
In this example, with reference to FIG. 2, it is possible to consider that the illumination of the magnetic field has induced current sheets (1) oriented along the axis Y (in the inspected zone), in which the presence of said emerging crack (2) has modified the path, creating by induction a field (3,4) detectable at the surface. There are several problems for ‘reconstructing’ the real geometry of the defect: the current sheets are different in each plane of depth Z, the defect can thus affect several, and the measuring device does not always give all of the components of the physical quantity radiated by the defect to the surface (magnetic or electric field, in the case of the ECI).